The first compiler hook is currently pre:compile and runs before compile (duh..), but after libs and assets have been copied.
I did an alloy.jmk file that searches for retina @2x images and uses ImageMagick to generate non-retina images if they don't already exist. Now because the pre:compile hooks runs after the assets have been copied, I have to search both the app and the Resources folder.
I'm sure there are other cases like this, for which a new pre:assets|files|copy hook would be useful. For the above use case, having this hook would also allow me to know the time of the previous compile (by getting the ctime of the Resources/alloy folder) and use this to detect updated retina files as well.